It has long been recognized that the early detection of flaws, both superficial and internal, in metals and similar materials is critical to the prevention of catastrophic failures and the consequences of such failures. For example, the integrity of welds, joints and load bearing structures can be severely compromised through the existence of flaws in their composite materials. Obviously the detection of flaws in such circumstances is of considerable concern from an economic, performance and safety standpoint. Flaws in materials utilized in the nuclear, avionics, and civil engineering construction industries can carry an even greater significance due to the relative impact that a material failure can have in these industries.
In an attempt to reduce the incidence of failures, materials scientists have developed a number of different methods to test for flaws. Flaws may consist of internal abnormalities in metals or materials, superficial irregularities including pitting and corrosion degradation, internal or external stress fractures, flaws from annealing or other heating and cooling processes, or a variety of other imperfections that may exist internally or superficially in a material. In some cases the existence of flaws is readily apparent from a visual inspection, however, in many instances a mere visual inspection is insufficient. This is particularly the cases where a flaw is completely internal and not detectable through conventional methods. These so called invisible flaws often tend to be the cause of the most damaging failures since failure occurs unexpectedly.
Devices such as high powered microscopes and X-ray machines have been developed to assist in the early detection of flaws of this nature. While each of these methods proved to be useful they also suffered from somewhat obvious limitations and inherent problems. Microscopes were useful to detect surface flaws but provide no assistance in locating internal abnormalities. X-ray machines proved to be difficult to operate, expensive and suffered from the limitations and concerns of devices operating with the use of a source of radiation.
To overcome the limitation of these methods of testing, a technique using eddy current excitation was developed. In this technique an eddy current probe coil is subjected to alternating current to create a time varying magnetic field. When the magnetic field is directed onto a metallic surface, eddy current are induced within the metal. These eddy currents then produce their own magnetic fields which have the effect of impeding the time varying magnetic field generated by the probe coil. Abnormalities or flaws in the metal tend to prevent the creation of eddy currents, hence having an effect on the impedance of the time varying magnetic field of the probe. Accordingly, through monitoring the impedance variation in the coil of the eddy current probe it is possible to detect the incidence of internal flaws within the metal.
Although eddy current probes were found to be a significant improvement in non-destructive testing over prior methods they suffered from their own inherent problems. Eddy current testing proved to be useful to detect cracks or flaws oriented generally perpendicular to the probe but did not reliably detect some forms of degradation and could not discriminate between combinations of different types of flaws that were found in the same position. As a result, the use of ultrasonic testing was introduced. In ultrasonic testing, an ultrasonic probe is positioned next to the testing material and the material is subjected to an ultrasonic beam which is reflected by flaws and analyzed. Such ultrasonic probes have been found to be particularly useful to detect flaws arranged horizontally relative to the beam and also are especially effective in detecting flaws where eddy currents are least effective. Unfortunately ultrasonic probes are also not without their limitations; most notably the relatively slow speed at which they must be moved along the testing material, and the need for direct contact, to maintain local sensitivity (when compared to eddy current probes) and their relative inability to detect flaws arranged generally parallel to the ultrasonic beam.
To combine the advantages of each of the eddy current and ultrasonic testing procedures, others have combined both an eddy current probe and an ultrasonic probe into a single device. Such devices have generally been restricted to very specific and limited uses, and particularly for testing the integrity of the walls of heat exchanger tubes in thermal generators in the electrical generation industry. Typically a carrier having both an eddy current and an ultrasonic probe fixed thereon would be inserted into a tube to inspect the tube's walls. The difficulty that is encountered in this application is that to examine the entire surface of the tube a number of eddy current and ultrasonic probes, each directed radially outward, have to utilized. In the alternative, a means of rotating the carrier has to be employed so that coverage of the entire surface is achieved. The use of a rotating carrier has the unfortunate disadvantage of either sacrificing local sensitivity of the ultrasonic testing or sacrificing the coverage available to the eddy current testing due to the variation in the effective rates at which each type of probe operates. Since the consequences of failing to detect a flaw are significant, it is usually opted to reduce coverage and maintain local sensitivity. The result of this is that the overall cost and time expended on testing increases significantly. Furthermore, the configuration of known and existing devices utilizing both eddy current and ultrasonic probes is limited to the testing of the walls of small tubes such as those in steam generators. Such devices do not lead themselves to applications beyond these types of limited uses.